1. Field of the Invention
The present invention relates to a discharge lamp device in which an arc tube elongates in the front of an insulating plug made of a synthetic resin, and particularly to a discharge lamp device which includes an arc tube body of a structure where shroud glass is welded and integrated with an arc tube and a discharging portion is enclosed by the shroud glass, and which has a structure where a rearward elongating portion of the arc tube elongating in the rear of the shroud glass is inserted and held into a front opening of an insulating plug made of a synthetic resin.
2. Description of the Related Art
Light emitted from a discharging portion contains ultraviolet rays in a wavelength region harmful to the human body and components (synthetic resins and metals) of a lighting device. In the conventional art, therefore, the arc tube body 2 is configured in the following manner. As shown in FIG. 29, cylindrical shroud glass 2b having a UV cutoff function is welded and integrated with an arc tube 2a to form a structure in which a discharging portion 2a1 is enclosed by the shroud glass 2b, thereby reducing ultraviolet rays contained in light emitted from the arc tube body.
A conventional discharge lamp device is structured in the following manner. A rearward elongating portion 2a2 of the arc tube which elongates in the rear of the shroud glass 2b is inserted into a front opening 1a of an insulating plug (insulating base) 1 which is made of a synthetic resin. The rear end side (a rear end portion of the shroud glass 2b) of the arc tube body 2 is fixingly supported by a fixing holder 3 which is secured to the front face of the insulating plug 1, and which is made of a metal. The front end side of the arc tube body 2 is supported by a lead support 4 which forward elongates from the insulating plug 1, and which is made of a metal.
The reference numeral 6 denotes a light blocking film for luminous intensity distribution control, which is disposed on an outer side face of the shroud glass 2b and is used for blocking part of light advancing toward an effective reflecting surface 8a of a reflector 8 to form a definite clear cut line.
In a reflection-type head lamp in which the above-mentioned conventional discharge lamp device is used as a light source, the effective reflecting surface 8a of the reflector 8 becomes cloudy.
According to studies by the inventors, it was assumed that such cloud is caused by UV components contained in leakage light from the arc tube body 2. Specifically, as indicated by the arrows in FIG. 29, part of light emitted from the discharging portion 2a1 of the arc tube leaks from the rearward elongating portion 2a2 and the like of the arc tube to irradiate the interior of the front opening 1a of the insulating plug 1. This leakage light contains ultraviolet rays in the wavelength region harmful to a synthetic resin. The insulating plug (PPS resin) is exposed to the ultraviolet rays to degrade and generate a sulfuric gas. The sulfuric gas is seemed to tarnish or cloud the effective reflecting surface 8a of the reflector 8.
Japanese Patent Laid-Open No. 2001-23427 discloses a conventional technique of solving the problem. In the technique, the area where a light blocking film for luminous intensity distribution control is disposed on shroud glass is extended to a root portion of the rear end side of the shroud glass (outer tube), whereby the amount of UV irradiation applied on the recess of a synthetic resin insulating plug so as to cope with the series of problems such as degradation of the resin due to UV.
In the above-described conventional art, the light blocking film for controlling the luminous intensity distribution which is required to have heat resistance and hence expensive is extended to the root portion of the rear end side of the shroud glass (outer tube). Therefore, a resulting discharge lamp device is further expensive. In order to form the light blocking film for luminous intensity distribution-control, a drying process is performed by baking the shroud glass at a high temperature of, for example, 900° C. Since the light blocking film is applied in a wide range, there arise problems in that the number of drying heaters must be increased, and that the time period required for the drying process is prolonged to increase the production cost.
Usually, the light blocking film for luminous intensity distribution control is colored in black. Since the light blocking film is extended to the root portion of the rear end side of the shroud glass (outer tube), the extended light blocking film is seen in black through a front lens, thereby impairing the appearance. In the field of a lamp, particularly, there is a recent tendency to control the luminous intensity distribution by a reflector and not to dispose steps for luminous intensity distribution control. Therefore, the light blocking film for luminous intensity distribution control is seen in black and in a large size through a front lens in which no step is disposed, whereby the appearance is largely impaired.
The inventors conducted experiments for ascertaining the effectiveness of blocking on the lighting time (the effectiveness such as that defects of, for example, a cloud on the effective reflecting surface 8a of the reflector 8 are not produced) while, as shown in FIG. 30, light is blocked in: only a rearward elongating portion A of the arc tube; only a rear end portion B (including the rear end) of the shroud glass facing the interior of the front opening 1a of the insulating plug; only a band region C between the fixing holder 3 and the light blocking film 6 for luminous intensity distribution control; only a region D which is below the light blocking film 6, and which corresponds to the vicinity of a bulb insertion hole; and all of the regions A to D. Results shown in FIG. 31 were obtained.
From the results, it was confirmed that, in the case where all of the regions A to D are blocked, the effectiveness is naturally attained, and the effectiveness is attained also in the case where the light blocking film is formed in only the rearward elongating portion A of the arc tube or in only the rear end portion B of the shroud glass facing the interior of the front opening 1a of the insulating plug 1. Furthermore, it was confirmed that the case where only A is blocked is more effective than that where only B is blocked. This seems to be caused by a phenomenon that the energy of leakage light from the arc tube is in proportion to the distance from the discharging portion. It was found that, in order to avoid defects such as UV degradation, blocking of leakage light from the rear end portion B of the shroud glass which is closer to the discharging portion than the rearward elongating portion. A is most effective. Namely, it is preferable to block leakage light from the rear end portion of the shroud glass before the light reaches the inner peripheral face of the front opening of the insulating plug.
The structure in the periphery of the front opening 1a of the insulating plug 1 will be considered. The metal fixing holder 3 which fixingly supports the shroud glass 2b is disposed in the peripheral edge portion of the front opening 1a of the insulating plug 1. Therefore, it has been considered that, when, for example, an economical light blocking member made of a metal is fixed to the metal fixing holder 3 so as to block leakage light from the rear end portion of the shroud glass 2b, therefore, the structure is simplified, and the production cost is not increased. The invention is based on this finding.
As discussed above, in the case where a UV blocking film is formed in all of the regions A to D, the effectiveness is naturally attained. It was confirmed that the effectiveness is attained also in the case where a UV blocking film is formed in only the rearward elongating portion A of the arc tube or in only the rear end portion B of the shroud glass facing the interior of the front opening 1a of the insulating plug 1.